1. Field of the Invention
This invention relates to an electronic flash device for a photographic camera and more particularly to a flash device which determines an aperture value through preliminary flashing and sets the light control level for main flashing according to this aperture value. The term "light control level" as used herein means a predetermined quantity of light received at which the main flashing is to be terminated.
2. Description of the Prior Art
It is well known that the quantity of flash light for an object to be photographed is reduced by diffusion of the flash light when a light diffusing panel is disposed in front of the flashing part of a flash photographing device for the purpose of altering a flash illuminating angle.
In a flash photographing device of the kind arranged to determine the aperture value of a photo-taking lens according to the measured level of a reflection light obtained by preliminary flashing and then to control main flashing in accordance with a light control level corresponding to the aperture value thus determined, the light quantity for an object to be photographed decreases due to diffusion of the light of main flashing when the illuminating angle of the main flashing is altered by providing a light diffusing panel in front of a main flashing part as mentioned above. Under such a condition, if a preliminary flashing action is performed in the same manner as in ordinary photographing to determine the aperture value of a photo-taking lens according to the measured level of a reflection light obtained by the preliminary flashing, insufficiency of light quantity might result from main flashing, depending on the distance to the object to be photographed, although the object is within an interrelated light control range with the light diffusing panel used. In the case of ordinary flash photographing with a flash device of, for example, a guide number 16 and with a film of sensitivity of, for example, ASA 100, flash photographing with controlled light quantity can be carried out for objects located at distances up to 4 m by adjusting the aperture value of a photo-taking lens to F4.0 according to a measured level of a reflection light obtained by preliminary flashing. However, when a light diffusing panel is disposed in front of a main flashing part to lower the guide number, for example, to 12, the maximum allowable distance to the object for control light photographing becomes 3 m. If the distance to the object is 3.5 m under this condition, the aperture value of the photo-taking lens would be adjusted to F4.0 in accordance with the measured level of a reflection light obtained by the preliminary flashing. Then, at the time of main flashing, flash photographing would be performed outside of the light controllable range and the photographing operation would result in an under-exposure photograph.